Effect of acetazolamide on stable carbon isotope fractionation in Chlamydomonas reinhardtii and Chlorella vulgaris

Abstract

The effect of extracellular carbonic anhydrase (CAex) on stable carbon isotope fractionation in algae is still unclear. The stable carbon isotope composition and algal growth in the presence and absence of the membrane-impermeable CA inhibitor acetazolamide were compared in Chlamydomonas reinhardtii and Chlorella vulgaris. The CAex of both algal species contributed about 9‰ of the stable carbon isotope fractionation and exhibited a dosage effect. Therefore, evidence in vivo that CAex leads to a larger carbon isotope fractionation of algae is presented. The isotopic composition of sedimented algal material is an indicator of paleoenvironmental conditions because of the well-documented effect of CO 2 concentration on marine algal carbon fractionation [1][2][3]. However, use of existing models for prediction of algal carbon isotope fractionation revealed a large deviation in some aquatic ecosystems [4]. This deviation, if not considered in prediction models, would affect the precision of predictions of paleoenvironmental CO 2 concentrations based on the carbon isotope composition.The deviation from the model-predicted isotope fractionation results not only from environmental factors, but also from some important physiological factors. Extracellular carbonic anhydrase action might be one of the main physiological processes that lead to the deviation. Carbonic anhydrase (CA; EC 4.2.1.1), a zinc-containing metalloenzyme, catalyzes the reversible interconversion between bicarbonate (HCO 3 − ) and CO 2 . The uncatalyzed, slow interconversion between CO 2 and HCO 3 − produced about 10‰ of the stable carbon isotope fractionation, whereas the interconversion in vitro catalyzed by CA had only 1.1‰ fractionation [5,6].The present study examines the effects of extracellular CA (CAex) on carbon isotope fractionation by comparison of the stable carbon isotope composition and algal growth with and without the membrane-impermeable CA inhibitor acetazolamide (AZ) in Chlamydomonas reinhardtii and Chlorella vulgaris. Evidence in vivo showed that extracellular CA leads to higher algal carbon isotope fractionation.